We have discovered a gene which putatively controls the time of the first cleavage division of preimplantation embryos. The gene, which we have called Ped for preimplantation embryo development, maps in the major histocompatibility complex (MHC). This is the first report of the involvement of the MHC in the control of development. In the mouse the H-2 linked Ped gene manifests itself as two functional alleles which control either fast or slow development. We propose to determine whether the trait is truly dichotomous, or if an array of alleles exists. We also propose to map the Ped gene within the H-2 complex to determine whether there is more than one Ped gene locus, and to evaluate the degree of dominance of Ped alleles. These goals will be accomplished by examining a series of inbred, congenic, recombinant, F1 hybrid, backcross, and F2 embryos for fast, slow, or intermediate patterns of development. Monoclonal antibodies to known H-2 antigens will be used as markers for H-2 linked Ped gene(s) in backcross and F2 embryos.